Multivibrator



Feb. 25, 1947. R, D, BROWN, JR 2,416,513

MULTIVIBRATOR vFiled Nov.. 25, 1942 ouTPl/T @m JVHW J4 l 74m 1 y M 5Patented Feb. 25, i947 UNITED STATES PATENT OFFICEl MULTIVDSRATORReynolds D. Brown, Jr., Blue BelLPa., assignoi` to l PhilcoCorporation,Philadelphia, Pa., a corporation of Pennsylvania fimplication November 25, 1942, 'serial Nene-6,913 e'claims. (cl. 25o-36) In the development of vacuum tube ampliiiers for use at highfrequencies attention was early directed to the screen grid tube as asuperior type of radio frequency amplifier. This superiority is inpart aresult of its low effective control-gridto-anode capacity which enablesits use, with good stability and high amplification, at frequenciesbeyond those for which the triode is suited. However, the insertion ofthe screening element produces a substantial increase in the tubes inputcapacity, i. e., in the direct interelectrode capacitance as measuredbetween the input grid, on the one side, and all of the remainingelectrodes on the other. In conventional pentodes this capacitance isabout double that for triodes. This high input capacitance is notordinarily serious in tuned amplifiers Where the input capacity of thetube may be used to tune the input circuit. But, as is Well known, inuntuned wide-bandqvsystems the increased input capacity seriously limitsthe maximum frequency at which the system is operable, and tends, athigh frequencies, to short circuit the signal source Withwhich it isdirectly f in shunt. j

It is a principal provide means for effectively `diminishing the inputcapacity of amplifier tubes having screen grids, and thereby to extendthe frequency range over which the tubes are useful.

It is another object of the invention to increase the maximum frequencyat which the multivibrator .type of oscillator is operable.

It is still another object of the invention to provide amultivibratorcapable of functioning 'at frequencies of the order ofseveral megacycles.

It is a further object of the invention to pro vide a signal amplitudelimiter of the multivibrator type capable of operating in the intervmediate frequency ampliiier channel of frequency modulation receivers. j

The'invention itself, as Well as other of the obiect of `4,thisinvention toV objects thereof, will be understood by those skilled inthe art from a consideration of the following detailed description andthe accompanying drawing, in which Fig. 1 is a schematic illustration ofa circuit utilizing the invention; and

Fig. 2 is a schematic illustration of a multivil brator circuitembodyingthe invention.

Reference is now made to Fig. 1 in which the invention is applied to asingle-stage vacuum tube Y amplifier.

Ignoringfor the moment, the pentode I, the amplifier stage will be seento consist of a `conventional amplifier circuit comprising the pentode 2together with an output load resistor 3. The signal to be amplified,derived from the source 4, i-s applied betweenthe cathode 5 and controlgrid 6 of the pentode 2, while the amp1i.

fled output signal may be derived from the anode circuit of the tubebyway of the coupling con-- denser 1. In addition to the elementsmentioned, the pentode includes a suppressor grid 8 which isconveniently connected to the cathode 5, and a Vscreen grid 9 whichwould ordinarily be connected to a source of iixed positivepotential(not shown) The input capacity of the pentode 2 consists largely of thecapacity between the control grid 6 on the one side and the` cathode 5and screen grid 9 on the other. Capacityalsoexists between the controlgrid B andthe anode and suppressor grid 3, but .since the latterelements are more remote from the control grid the contributions ofVthese elements are of .less importance.

At the higher frequencies a considerable charging current flows into thecontrol grid 5 from the signal source 4, .and this charging current islargely due tothe physical proximity ofthe cathode 5, here shownconnected to ground,l andto the screen grid 9, which in conventionalpractice l is at ground potential so far as the radio frequency inputsigna-1 is concerned. As the Vsignal frequency increases in magnitude,the charging current also increases,'thus placing an abnormally heavyload upon the signal source 4, and eifectively short-circuiting theinput circuit ofthe pentode'2. Y f According to the present invention,the interelectrode capacity effects between the input grid B and thescreen grid 9 are substantially nullified by the application to thescreen grid 9 of a signal which is similar in frequency, phase, andmagnitude to the signal applied to the input grid 6. This 'may' beaccomplished through theagency of an auxiliary pentode I. In theembodiment ofthe invention illustrated in Fig. 1, output signal from thepentode 2 is derived from the load resistor 3 and impressed on the inputgrid I0 of pentode l by way of the coupling condenser The output signalof the auxiliary pentode 1I, as taken from a point on the anode loadresistor I2, is applied to the screen grid 9 of pen- 5 tode 2 by way ofthe conductor I3. Since a phase reversal occurs.; in ,each-l of thepentodesfithe sig-J nal applieditoathe'screengrid 9 is in ...the isame"z phase as the original signal applied to the control grid 6, and by asuitable choice of tap locations on .2.103n

the impedances 3 and I2 the magnitude of the signal supplied to thescreen grid 9 may be made equal to that present on the control grid.When .,Y the alternating voltages appearingcnthegrigdsaz `6 and 9 areequal in magnitude. and of like phase, 151Ml far beyond itsnormal'range, Y

it is evident that no charging. current can flow from they control grid6 to the-screelr-gridandai thus one cause of the objectionablechargingcune; rent is eliminated. However, it is also within the scopefvthe invention izo-'compensate wholly or..20 in-tpartior thecapacity"ieiliects between the con`` complis'he'd byY supplyingl'tol-the screen* grid' 9 a signal of greater magnitude thanthat-'presenten the control grid 6. f Under theseconditi'ons charg- 25.ing current will ow fromfthe"screengri'df9,asa source; lto the controlgrid 56,. l'and by suitably adjusting the magnitude "of" the? voltagesupplied=by'*"" the" pentode'v I"v it is readilyfp'ossible-to'compensate for"k the charging current-i takenffbythe control '30gridato-cathode?circuit'cf the fpento'deZ, v' Sirr'ii'' larlyfit ispossibletocompensateforth charging current 'drawn by other Joffth-`:electrodesif or 'by the distributed-capacity associated r withl the'Fcon# ductors'connectedito thegri'd. 35 Asidev from :itsdesiredcapacitycompensatingfI" effect; th 'applicationoof the.ini-'phase r'signal f'tof the screen'- gridl 9 producesinoisubstantialchange-- in-the operationfof thezfamplifie'nfrand no vdis-ri`advantageous "veiectsff 'are '-Sintroduced'.v This" is largely be'causelthe screenfgridetoeanode'fmutual L conductan'ceofthe'itube issmallclomparedzt'o thei control: grid-toganodemutualr-conductance; and1consequentlyffthecontrolaeffectof vthescreen?. is i negligible. .At1lowfsign'al requenciesiwhereftube. :45 input capacity 1' would@ havenolf effect 1on1v circuit iperformance; theapplication;.ofil the: signalvoltjagefto the :screenr'S hasfsnoisubstantial .-ieect ioni. ffth'epgain 'of .'thitubeie However,'t;at;th'e1h glrsignal.: I frequencieswheretrgain:wouldinormallyffall ofi L: rapidly :witlrfrequencyzthe'zpresent;circuit tendszto reduceanthe: effectivesrzinputu-,capaciw and* toil: maintain 'iavorablef operating-5:onditions;rwithin. 'the'amplie circui Referringnowftozitheedetailsfsciithevcircuit::ofi: g

Figi T1, .it iwi-1libeznotedffthatathe input? signal :to .1 5" the.auXiliarymentode it is derived .from an Y inter-1f mediatepointronffthesresistor, if This practice-f-zv `is .preferred because thro-ughthis#,expedient the f e distributed capacities'fto ground of -the.fcoupling- 'f condensenl I yand the?leadfwiremassociatedftherewitlfarenot connecteddirectly; in shunt with-.the anodev circuit.i'offtithearripliiiersipentodel 2, 'andnz this Sway th'e`output-'icapacity"1ofathis pentodei is f not adversely effected by thepresence ofthe265 `aux-iii'aryrpentode'lz Infpracticfit Willorma'lly Ibedesira-ble toL employ intermediate "taps'onf bo'th f1 l th'resistorsandu |32 (asshown) pinv order to ree strict the .over-alhg'air of the;auxiliaryiypentode' circuitjtoj; that:;whijch .is just.jnecessary-"totcom' p 70 pensatelforcthe.capacitygeffects hereinbefore"de' scribed aJns-.the #circuit shown, plate, v.and screen. fpotentialifor thepentodeol., .andiscreenpotentiali.. for Lfthe...pentode .2, .is ...derived. .from a .common q source B+. This is aconvenient Connectfm, bli 75 'tot obviously separate sources may beemployed if desired.

In general it is not difficult to design an auxiliary pentode circuitcapable of providing, even at very high frequencies, a signal forapplication to the screen grid 9 which is equal to or somewhatgreaterfthanthe Asignal applied toEv the input grid 6. s This isbecausethe overall gain `ofithe auxiliary pentode circuit need not belarge, being, in

igeneral, substantially less than unity, or of the order of unity,depending upon the gain of the amplieapentode 2. Under thesecircumstances, andibyproyiding theauxiliary pentode with lowinip'egdanceinput-andiutputcircuits, the operatinif;;requeneyz.ci.;thpentode I may be extended Referenceishowmade to Fig..2in .which the v invention..is .applied to a multivibrator circuit whichis 'adapted for use v.asan amplitude limiter the anode-4 2 Iff' 'of'.-tuberfl 5 bytiwayiior Pa couplingff'f condenser'2.31.'-`- The'tubeM'nay be provideclwith'v a suitablegri'dilea'kf.resistenrZAJanrl'anfi'ariode load' circuit cc-mprisingthseriallyconnected resistors 25 andE SimilarlythetubelS may'beprovidedwith a grid1l'eak1'213 and anodeloaii resistors V28 and" Z Aniinputsig-na'lofrom 'a so.urce"30''mayl if be applied". tofthe tubelf ilmth-rough fan l input'grid" 3|? The correspondingyinputgridIZ'iofthe'tube'v Y l 5f may be" connected ltofground, `but if"af balanced' source 1 of signalls provided-,ifthesignal may supplied'.tov both :finput '.lgrids finbpushepull' rela;` tion, LTheoutputLSignaldfthemultivibrator maybetake'nfrom either# 'ofthe anodesfZtl-lori.'l;4'or if :desired the 1 output fsignalwmay. -`bewaken-from the" anodesin'ipushepull rela-tio'n.- asf shown; the?. push-pull connection' givingktwicethe output prol1 vided by :the'singlelside"connectionzi1Thf-vacuumvtubewl'g infaddition Ato its conven= tional f function'`finif '-the'l multivibrator; lis related tothe vacuum tube I4 in muchthe same'wayf" asf-"the" auxiliaryyacuum'iube* iI 'of-Fig v'1 isrei'ateclrftb:itgassociated va'cuum'tubeZ. Thus Fi'gfZ utputs'igna'lfrcmth'e' tube I'5is derived' from thevv junction fbetween'theanod'e'fload esisn'ftors 28A and 29, and is '-*applie'drby'f way offthe"conductor '33" to" Vthef =screerr` grid lernent 34 of tube I4.`Similarlyioutput-'sigallfromthefltube t I4 is derivedfromthejunctio'nofthe'anode load resistors '25'and" 26 and is appliedbytwayA of :the Y u conductor 35'toith'e'-screen'grid`-36of1thetube I5.In' additongto thev he'renbefore 'described'physe ical circuitvelementsizthe.schematicoffFig 2 also. shows, .in .dashed lines," fthe"effective capacities i: associatedgwith the-.tubeelectrodes.: Tlis the'capacityin shunt 'with`the grid leak* resist'o'r2il` represents the sum01 .the eiectivcapa'cties from thJ'grid I'8`to`thscreengridi34,"frm`the' grid plate 2| of tube I5 to the suppressor grid31 of tube I5, and the load capacity of the tube I5. The capacity shownin shunt with the load resistor 28 represents the capacity lfrom thegrid I8 of tube I4 to the screen grid 34 of tube I4. This capacity isincluded in the capacity in shunt with the grid leak resistor 24, but itis here isolated. The capacity shown in shunt with the load resistor 29represents the capacity from the screen grid 34 of tube I4 to thesuppressor grid 38, and to the input grid 3| of the tube I4. Thecapacities shown in shunt with the grid leak resistor 21 and the loadresistors 25 and 26 correspond to those above mentioned.

In the preferred form of the invention. the time constants of the sixR.C. circuits, which include the resistors 24 to 29 inclusive. arepreferably made equal by a suitable selection of the several resistors.Under these conditions the effective capacities of the circuit disappearat all frequencies for which the equality is substantial. Even if thispreferred adjustment is ignored with respect to the grid leak resistors24 and 21, it will be advantageous to adjust the time constants of theR.C. circuits which include the resistors 25, 26, 28 and29.

In the design of amplifier and multivibrator circuits constructed inaccordance with the principles of the invention, the followingrelations, while not advanced as limitations to the invention, will behelpful. Taking Cg as the interelectrode capacity between the grid I8and the screen grid 34, and taking Cs as the capacity between the screengrid 34 and all the other electrodes exclusive of grid I8, in thepreferred embodiment Rea R25-t' Rza is greater than zero, but less thanMoreover, in general, the lower the values of R25 and R26 the higher theeffective frequency of operation, but the higher the level of theminimum signal which must be applied to reach the limit effectively. Thecorresponding resistors associated with the second tube I5 may besimilarly evaluated.

The multivibrator circuit illustrated in Fig. 2 may advantageously beemployed Wherever a, multivibrator capable of generating a signal ofhigh fundamental frequency is required. The multivibrator circuit shownis capable of being synchronized with signals from the source 30 whosefrequency is well in excess of one megacycle` a frequency which formerlyrepresented the upper practical limit for multivibrators of conventionaldesign.

The multivibrator of Fig. 2 is particularly well adapted for use as asignal amplitude limiter in the intermediate frequency channel of afrequency modulation radio receiver. The conventional form ofmultivibrator, on the other hand, is largely inoperative at theintermediate frequency generally used in frequency modulation receivers.Even were the intermediate frequency made so low that this considerationdid not apply, the deviation of the frequency-modulated carrier wouldcause it to reach frequency regions in which the conventionalmultivibrator is unreliable. The present invention however eliminatesthe difiioulties previously considered inherasians i the control gridand cathode electrodes of saidv ent in the multivibrator, and provides anonresonant limiter system which is readily synchronized with thefrequency-modulated intermediate frequency signal. It also provides anamplifier of simple construction capable of high gain while limiting theoutput, in addition to providing a push-pull source of limited signalsusing a single side input signal.

Although the invention has been described and illustrated withparticular reference to certain preferred embodiments, it should beunderstood that numerous alterations and modifications may be madewithin the scope of the invention as defined in the appended claims.

I claim:

l. A multivibrator circuit adapted for operation at high frequencies,comprising a pair of vacuum tubes, each having at least a cathode,control grid, screen grid and anode, a load resistor connected to eachof said anodes, said cathodes, control grids and anodes beinginterconnected to provide multivibrator operation, means for couplingthe screen grid of the first of said tubes to an intermediate point onthe load resistor of the second of said tubes, and means for couplingthe screen grid of said second tube to an intermediate point on the loadresistor of. said rst tube.

2. A multivibrator circuit as claimed in claim 1, wherein at least oneof said tubes is provided with an auxiliary grid, and wherein asynchronizing signal is applied to said grid for controlling the periodof oscillation of said circuit.

3. A multivibrator circuit adapted for operation at highfrequencies,'comprising a pair of vacuum tubes, each having at least acathode, control grid, screen grid and anode, a first pair of loadresistors serially connected in the anode circuit of the first of saidtubes, a second pair of load resistors serially connected in the anodecircuit of the second of said tubes, a first condenser connected betweenthe anode of said first tube and the control grid of said second tube, asecond condenser connected between the anode of said second tube and thecontrol grid of said rst tube, a connection between the screen grid ofsaid first tube and the junction of said second pair of load resistors,and a connection between the screen grid of said second tube and thejunction of said first pair of resistors.

4. A multivibrator circuit as claimed in claim 3, characterized in thatthe time constants of the circuits formed by the said resistors and thecapacities in shunt therewith are substantially equal.

5. A multivibrator circuit as claimed in claim 3, wherein grid leakresisto-rs are connected between tubes, and characterized in that thetime constants of the circuits formed by the said grid leak and loadresistors and the capacities in shunt therewith are substantially equal.

REYNOLDS D. BROWN, JR.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,033,274 Burton Mar, 1l), 19361,986,331 Farnsworth Jan. 1, 1935 1,941,393 Farnham Dec. 26, 19332,145,368 OBrien Jan. 31, 1939 2,299,366 Ziel Oct. 20, 1942

